(a) Field of the Invention
The invention relates to selective surfaces for photothermal solar collectors, a process of preparing these selective surfaces as well as solar collectors consisting of such surfaces. More specifically, the invention deals with the preparation of selective surfaces for photothermal solar collectors by dry oxidation at high temperature.
(b) Description of Previous Methods
There are numerous techniques for preparing surfaces for photothermal solar collectors. Most of these techniques are based on well known electrochemical methods or rely simply on the use of absorbing paints. Other methods, which are much more costly but which give also a better performance, rely on various coating processes from the vapor phase.
On the other hand, it is well known that it is relatively easy for the current photothermal collectors to convert substantially the entire solar energy into heat and this can be obtained only if the absorbing material cost is acceptable. However, this is often associated with an important disadvantage with respect to the global thermal efficiency. Depending on its degree of heating, the absorbing surface will have a tendency to re-emit more and more energy in the form of infrared radiation. This loss is even higher when the operating temperature of the collector is high and its coefficient is large.
The problem encountered by the radiation of the collector in the infrared, although important, is resolved with the preparation of materials which are called selective with respect to the solar radiation, i.e. whose coefficient of absorption in the range of the solar spectrum is close to unity and whose emissivity in the infrared is, on the contrary, quite low. Such materials are already known. However, their manufacturing cost is generally quite high, and in certain cases, they cannot sustain high operating temperatures. By including selective and non selective materials, photothermal absorbing surfaces may be classified into four important types:
(1) First, the non selective surfaces with substantially equal coefficients of absorption and emission. This is the behavior which is more often attributed to the various materials. The surface has a coefficient of absorption which is close to one, and is, for example, made of carbon black in an organic binder.
(2) Among the selective surfaces, those formed of a semi-conductor thin film at the surface of a polished metal. The thin film has a high coefficient of absorption within the visible spectrum and is transparent in the infrared range. At the interface, the metal is characterized by a very low emissivity in the visible range as well as in the infrared. The thickness of the film is at the most a few microns. The transfer of energy from the semi-conductor to the metal is mainly carried out by phonons and, less extensively, by photoelectric conversion.
(3) The dielectric materials in multiple layers form another category of selective surfaces. High absorption is achieved by multiple interference, a principle widely used in optics, except that, in the present case, the substrate is formed of a polished metal.
(4) It is also possible to produce a selective surface by acting directly on the microscopic morphology of the absorbing surface. The surface can be entirely metallic, but its morphology is such that the absorption of the solar radiation is almost complete while the same surface continues to benefit from a low emissivity in the infrared. This phenomenon may be explained by analogy with the phenomenon of cavity in the electromagnetic theory. Obviously, noble metals are too expensive in practice. However, the preceeding phenomenon is modified when current metals, always oxidized in surface, are used.
For a good review of the materials susceptible to be used as photothermal solar collectors, reference can be made to Walter F. Bogarts and Carl M. Lambert, Review-Materials for Photothermal Solar Energy Conversion, Journal of Materials Science 18 (1983) 2847-2875. It will be noted, in particular, that in the case of nickel, when the absorbing layer is constituted by Ni, NiO.sub.x, the absorption coefficient is 0.80 when the material is obtained by evaporation under vacuum and simultaneous thermal oxidation. In this case, the emissivity at 200.degree. C. is 0.1. When the material is obtained by electrodeposition followed by anodization, the absorption coefficient is 0.95 while the emissivity is 0.3 at 300.degree. C. Consequently, the optical properties of these materials seem satisfactory. However, their preparation is rather complex and their optical properties degrade during further oxidation.
In D. L. Douglas and R. B. Pettit, Solar Energy Materials 4, 1981, 383-402, the formation of an oxide layer on a metal in order to produce selective surfaces is described. The method merely consists of oxidizing a metallic sheet, in air or oxygen, in order to produce a compact layer of oxides, without the formation of discs or other surface roughnesses. The measured absorptions were up to 0.84 and the emissivity at 100.degree. C. between 0.06 and 0.39. This is a theoretical study without commercial application.